The gas wave ejector (GWE) is an efficient gas wave equipment using pressure waves to realize energy exchange. In this paper, a theoretical analysis of the limitation of application range and the factors affecting the performance of GWE was carried out by numerical simulation. And a complete experimental system including an adjustable GWE was employed to obtain the specific performance values in various working conditions. Such theoretical analysis showed that the device became inapplicable with a relatively high driving pressure ratio, resulting from the generation of the supersonic flow at the outlet end of the passages. A relatively high supercharging ratio also limited the equipment application because of the weakening of the reflected expansion waves and the enhancement of the reversed compression wave. Furthermore, the mixing, vortex, viscosity and other flow losses could also affect the equipment performance. Then, a complete performance map indicating the specific performance values in the application range was obtained by plenty of experiments. The performance map proved that GWE had excellent efficiency and broad applicability especially as the driving pressure ratio was lower than 2.6. The results are significant for practical application and performance improvement of GWE.

The expansion of renewable energy generation must go hand in hand with measures for reliable energy supply and energy storage. A combination of hydrogen and oxygen as storing media provided from electrolysis at high pressure and zero emission power plants is a very promising option. The Graz cycle is an oxy-fuel combined power cycle that can operate with internal H 2 /O 2 combustion and steam as working fluid. It offers thermal efficiencies up to 68.5% (lower heating value - LHV). This work applies a second law analysis to the Graz cycle and determines its exergetic efficiency. Exergy destruction is broken down to the cycle's components, thus providing insights on the location and magnitude of the cycle's inefficiencies. A sensitivity analysis identifies the cycle's exergetic and energetic efficiency as a function of representative parameters, offering an approach for future improvements. The combination of the cycle with an electrolysis plant is subsequently analyzed as an electric energy storage system. The round trip efficiency of the storage and back conversion system is computed by taking into account the additional compression of the reactants. As part of this analysis, the effect of the electrolyzer's operational pressure is studied by comparing several commercial electrolyzers.

Converting existing diesel engines to natural-gas spark-ignition (SI) operation can reduce the dependence on oil imports and increase energy security. NG-dedicated conversion can be achieved by the addition of a gas injector in the intake manifold and of a spark plug in place of the diesel injector. Previous studies indicated that lean-burn NG inside the traditional diesel chamber (i.e., a bowl-in-piston geometry) is a two-stage combustion (i.e., a fast burn inside the bowl followed by a slower burn inside the squish). However, a triple-peak apparent heat release rate was seen at specific operating conditions (e.g., advanced spark timing at medium load and engine speed), suggesting that one of the two combustion stages may separate again. Specifically, the burn inside the squish region divided in two events before and after top-dead-center. This was due to the different flow motion inside the squish during the compression stroke compared to the one in the expansion stroke, which affected the combustion environments. The result was the apparition of two close peaks in pressure trace, which suggest larger gradients in pressure and temperature than at a more delayed spark timing. In addition, the phasing and magnitude of three peaks of the heat release changed cycle-to-cycle. As an advanced spark timing is the usual strategy used in lean-burn SI combustion, understanding phenomena such as the one presented here can be important for reducing engine-out emissions and increase engine efficiency.

Heavy-duty compression–ignition (CI) engines converted to natural gas (NG) spark ignition (SI) operation have the potential to increase the use of NG in the transportation sector. A three-dimensional (3D) numerical simulation was used to predict how the conventional CI combustion chamber geometry (i.e., re-entrant bowl and flat head) affects the combustion stability, performance, and emissions of a single-cylinder CI engine that was converted to SI operation by adding a low-pressure gas injector in the intake manifold and a spark plug in place of the diesel injector. The G-equation based 3D computational fluid dynamics (CFD) simulation investigated three different combustion chamber configurations that change the size of the squish region at a constant compression ratio (CR) and a clearance height. The results show that the different flame propagation speeds inside and outside the re-entrant bowl can create a two-zone combustion phenomenon. Moreover, a larger squish region increased the flame burning speed, which decreased late-combustion duration (DOC). All these findings support the need for further investigations of the combustion chamber shape design for optimum engine performance and emissions in CI engines converted to NG SI operation.

In an internal combustion engine, the centrifugal compressor is placed upstream of the inlet manifold and therefore, it is exposed an unsteady flow regime caused by the inlet valves of the cylinder arrangement. This valve motion sets a pulsating state at the compressor exit, having greater influence when the operation is near the surge margin of the compressor. This paper presents the experimental results of the evaluation of the surge dynamics on a compressor with induced downstream pulsating flow. Different pulsation levels are achieved by the variation of three different parameters on the induced pulse: pulse frequency, amplitude, and system storage volume (plenum). Each pulse parameter was evaluated independently in order to assess its effect on the compressor stability limit. The main effect on the surge margin of the compressor was found to be due to the presence of a storage volume in the system for all cases (steady/pulsating condition) and at all frequencies. It was found that the magnitude of the pulse frequency determines the hysteresis behavior of the system that leads to a phase difference between the convected terms and the acoustic dominated terms, and therefore this affects the onset of flow instability, surge, in the compression system under study.

Wet compression is a strategy adopted to increase the power output of gas turbines, with respect to dry conditions, usually also incrementing the operating range of the compressor. However, stall and surge are two aerodynamic instabilities which depend on many factors, and they are expected to occur even in wet compression at low flow rates. Despite the many studies carried out in the last 80 years, literature does not offer many works concerning these instability phenomena in wet compression. In this paper, an experimental analysis of stall and surge in wet compression conditions is carried out on an axial-centrifugal compressor installed in an existing test rig at the Engineering Department of the University of Ferrara. The intake duct was implemented with a water injection system (WIS) which allows the uniform mixing of air and water before the compressor inlet. The control and data acquisition system of the test bench was updated with new hardware and software to obtain faster data sampling. Transient and steady-state tests were carried out to make a comparison with the experimental results in dry conditions. The analysis was carried out using traditional thermodynamic sensors, by means of both classic postprocessing techniques and cyclostationary analysis. The aim is to (i) evaluate the influence of wet compression on the stable performance of the compressor, (ii) qualitatively identify the characteristics of stall and surge in wet compression, and (iii) demonstrate the reliability of cyclostationary analysis in wet compression conditions for stall and surge analysis.

In order to investigate the performance and emissions behavior of a high compression ratio compression ignition (CI) engine operating in partially premixed charge compression ignition (PPCI) mode, a series of experiments were conducted using a single-cylinder engine with a high-pressure rail fuel injection system. This included a moderately advanced direct injection strategy to attempt PPCI combustion under low load conditions by varying the injection timing between 25 deg and 35 deg before top dead center (BTDC) in steps of 2.5 deg. Furthermore, during experimentation the fuel injection pressure, engine speed, and engine torque were kept constant. Performance parameters and emissions were measured and analyzed using a zero-dimensional heat release model. Compared to the baseline conventional 12.5 deg BTDC injection, in-cylinder pressure and temperature were higher at advanced timings for all load conditions considered. Additionally, NO x , PM, CO, and total hydrocarbon (THC) were higher than conventional results at the 0.5 N·m load condition. While PM emissions were lower, and CO and THC emissions were comparable to conventional injection results at the 1.5 N·m load condition between 25 deg and 30 deg BTDC, NO x emissions were relatively high. Hence, there was limited success in beating the NO x -PM trade-off. Moreover, since the start of combustion (SOC) occurred BTDC, the resulting higher peak combustion pressures restricted the operating condition to lower loads. As a result, further investigation including exhaust gas recirculation (EGR) and/or variance in fuel cetane number (CN) is required to achieve PPCI in a high compression ratio CI engine.

Deep surge is a violent fluid instability that occurs within turbomachinery compression systems and limits the low-flow operating range. It is characterized by large amplitude pressure and flow rate fluctuations, where the cross-sectional averaged flow direction alternates between forward and reverse. The present study includes both measurements and predictions from a turbocharger centrifugal compressor installed on a gas stand. A three-dimensional (3D) computational fluid dynamics (CFD) model of the compression system was constructed to carry out unsteady surge predictions. The results included here capture the transition from mild to deep surge, as the flow rate at the outlet boundary (valve) is reduced. During this transition, the amplitude of pressure and flow rate fluctuations greatly increase until they reach a repeating cyclic structure characteristic of deep surge. During the deep surge portion of the prediction, pressure fluctuations are compared with measurements at the corresponding compressor inlet and outlet transducer locations, where the amplitudes and frequencies exhibit excellent agreement. The predicted flow field throughout the compression system is studied in detail during operation in deep surge, in order to characterize the unsteady and highly 3D structures present within the impeller, diffuser, and compressor inlet duct. Key observations include a core flow region near the axis of the inlet duct, where the flow remains in the forward direction throughout the deep surge cycle. The dominant noise generation occurs at the fundamental surge frequency, which is near the Helmholtz resonance of the compression system, along with harmonics at integer multiples of this fundamental frequency.

This research evaluated the operational conditions for a diesel engine with high compression ratio (CR) converted to spark ignition (SI), under stable combustion conditions close to the knocking threshold. The main fuel used in the engine was biogas, which was blended with natural gas, propane, and hydrogen. The engine limit to test the maximum output power was using the knocking threshold; just below the knocking threshold, the output power and generating efficiency are the highest for each blend. Leaner mixtures increased the engine knocking tendency because the required increase in the % throttle reduced the pressure drop at the inlet stroke and increased the mixture pressure at the end of the compression stroke, which finally reduced the ignition delay time of the end gas and increased the knocking tendency of the engine for all the blends. Therefore, the output power should be decreased to operate the engine below to the knocking threshold. Purified biogas achieved the highest output power and generating efficiency compared with the other blends and the original diesel operation; this blend was operated with five equivalence ratios. Purified biogas exhibits an optimal balance between knocking resistance, low heating value, flame speed, and energy density.

Low temperature and dilute homogenous charge compression ignition (HCCI) and spark-assisted compression ignition (SACI) can improve fuel efficiency and reduce engine-out NO x emissions, especially during lean operation. However, under lean operation, these combustion modes are unable to achieve Environmental Protection Agency (EPA) Tier 3 emissions standards without the use of lean aftertreatment. The three way catalyst (TWC)-SCR lean aftertreatment concept investigated in this work uses periodic-rich operation to produce NH 3 over a TWC to be stored on a selective catalytic reduction (SCR) catalyst for NO x conversion during subsequent lean operation. Experiments were performed with a modified 2.0 L gasoline engine that was cycled between lean HCCI and rich SACI operation and between lean and rich spark-ignited (SI) combustion to evaluate NO x conversion and fuel efficiency benefits. Different lambda values during rich operation and different times held in rich operation were investigated. Results are compared to a baseline case in which the engine is always operated at stoichiometric conditions. SCR system calculations are also presented to allow for comparisons of system performance for different levels of stored NH 3 . With the configuration used in this study, lean/rich HCCI/SACI operation resulted in a maximum NO x conversion efficiency of only 10%, while lean/rich SI operation resulted in a maximum NO x conversion efficiency of 60%. If the low conversion efficiency of HCCI/SACI operation could be improved through higher brick temperatures or additional SCR bricks, calculations indicate that TWC-SCR aftertreatment has the potential to provide attractive fuel efficiency benefits and near-zero tailpipe NO x . Calculated potential fuel efficiency improvement relative to stoichiometric SI is 7–17% for lean/rich HCCI/SACI with zero tailpipe NO x and −1 to 5% for lean/rich SI with zero tailpipe NO x emissions. Although the previous work indicated that the use of HCCI/SACI increases the time for NH 3 to start forming over the TWC during rich operation, reduces NH 3 production over the TWC per fuel amount, and increases NH 3 slip over the SCR catalyst, if NO x conversion efficiency could be enhanced, improvements in fuel efficiency could be realized while meeting stringent tailpipe NO x standards.

The introduction of natural gas (NG) in the road transport market is proceeding through bifuel vehicles, which, endowed of a double-injection system, can run either with gasoline or with NG. A third possibility is the simultaneous combustion of NG and gasoline, called double-fuel (DF) combustion: the addition of methane to gasoline allows to run the engine with stoichiometric air even at full load, without knocking phenomena, increasing engine efficiency of about 26% and cutting pollutant emissions by 90%. The introduction of DF combustion into series production vehicles requires, however, proper engine calibration (i.e., determination of DF injection and spark timing maps), a process which is drastically shortened by the use of computer simulations (with a 0D two zone approach for in-cylinder processes). An original knock onset prediction model is here proposed to be employed in zero-dimensional simulations for knock-safe performances optimization of engines fueled by gasoline-NG mixtures or gasoline-methane mixtures. The model takes into account the negative temperature coefficient (NTC) behavior of fuels and has been calibrated using a considerable amount of knocking in-cylinder pressure cycles acquired on a Cooperative Fuel Research (CFR) engine widely varying compression ratio (CR), inlet temperature, spark advance (SA), and fuel mixture composition, thus giving the model a general validity for the simulation of naturally aspirated or supercharged engines. As a result, the auto-ignition onset is predicted with maximum and mean error of 4.5 and 1.4 crank angle degrees (CAD), respectively, which is a negligible quantity from an engine control standpoint.

Gas property prediction is necessary for proper design of compressors. Equations of state are utilized to predict the thermo-physical gas properties needed for such calculations. These are semi-empirical models that allow the calculation of thermodynamic properties such as density, enthalpy, and speed of sound of gas mixtures for known pressures and temperature. Currently, there is limited or no data publically available to verify the results of these equation of state calculations for the range of pressures, temperatures, and gas compositions relevant to many oil and gas applications. Especially for isentropic enthalpy head (i.e., the enthalpy rise along constant entropy lines), which is a critical parameter required to accurately design and performance test compressors, limited public domain data are available for equation of state validation. In this paper, a method and test apparatus is described to measure compression enthalpy rise directly. In this apparatus, a test gas is compressed using a fast acting piston inside an adiabatic autoclave. Test results are then corrected using calibration efficiencies from a known reference gas compression process at a similar Reynolds number. The paper describes the test apparatus, calibration, measurement methodology, and test results for one complex hydrocarbon gas composition at elevated temperatures and pressures. An uncertainty analysis of the new measurement method is also presented and results are compared to several equations of state. The results show that commonly used equations of state significantly underpredicted the compression enthalpy rise for the test gas case by more than 6%.

The present paper represents a small piece of an extensive experimental effort investigating the dual-fuel operation of a light-duty spark ignited engine. Natural gas (NG) was directly injected into the cylinder and gasoline was injected into the intake-port. Direct injection (DI) of NG was used in order to overcome the power density loss usually experienced with NG port-fuel injection (PFI) as it allows an injection after intake valve closing. Having two separate fuel systems allows for a continuum of in-cylinder blend levels from pure gasoline to pure NG operation. The huge benefit of gasoline is its availability and energy density, whereas NG allows efficient operation at high load due to improved combustion phasing enabled by its higher knock resistance. Furthermore, using NG allowed a reduction of carbon dioxide emissions across the entire engine map due to the higher hydrogen-to-carbon ratio. Exhaust gas recirculation (EGR) was used to (a) increase efficiency at low and part-load operation and (b) reduce the propensity of knock at higher compression ratios (CRs) thereby enabling blend levels with greater amount of gasoline across a wider operating range. Two integral engine parameters, CR and in-cylinder turbulence levels, were varied in order to study their influence on efficiency, emissions, and performance over a specific speed and load range. Increasing the CR from 10.5 to 14.5 allowed an absolute increase in indicated thermal efficiency of more than 3% for 75% NG (25% gasoline) operation at 8 bar net indicated mean effective pressure (IMEP) and 2500 rpm. However, as anticipated, the achievable peak load at CR 14.5 with 100% gasoline was greatly reduced due to its lower knock resistance. The in-cylinder turbulence level was varied by means of tumble plates (TPs) as well as an insert for the NG injector that guides the injection “spray” to augment the tumble motion. The usage of TPs showed a significant increase in EGR dilution tolerance for pure gasoline operation, however, no such impact was found for blended operation of gasoline and NG.

Experimental work on reactivity-controlled compression ignition (RCCI) in a small-bore, multicylinder engine operating on premixed iso-octane, and direct-injected n-heptane has shown an unexpected combustion phasing advance at early injection timings, which has not been observed in large-bore engines operating under RCCI at similar conditions. In this work, computational fluid dynamics (CFD) simulations were performed to investigate whether spray–wall interactions could be responsible for this result. Comparison of the spray penetration, fuel film mass, and in-cylinder visualization of the spray from the CFD results to the experimentally measured combustion phasing and emissions provided compelling evidence of strong fuel impingement at injection timings earlier than −90 crank angle degrees (deg CA) after top dead center (aTDC), and transition from partial to full impingement between −65 and −90 deg CA aTDC. Based on this evidence, explanations for the combustion phasing advance at early injection timings are proposed along with potential verification experiments.

In this paper, pilot-ignited high pressure dual-fuel combustion of a natural gas jet is investigated on a fundamental basis by applying two separate single-hole injectors to a rapid compression expansion machine (RCEM). A S hadowgraphy system is used for optical observations, and the combustion progress is assessed in terms of heat release rates (HRRs). The experiments focus on the combined influence of injection timing and geometrical jet arrangement on the jet interaction and the impact on the combustion process. In a first step, the operational range for successful pilot self-ignition and transition to natural gas jet combustion is determined, and the restricting phenomena are identified by analyzing the shadowgraph images. Within this range, the combustion process is assessed by evaluation of ignition delays and HRRs. Strong interaction is found to delay or even prohibit pilot ignition, while it facilitates a fast and stable onset of the gas jet combustion. Furthermore, it is shown that the HRR is governed by the time of ignition with respect to the start of natural gas injection—as this parameter defines the level of premixing. Evaluation of the time of gas jet ignition within the operability map can therefore directly link a certain spatial and temporal interaction to the resulting heat release characteristics. It is finally shown that controlling the HRR through injection timing variation is limited for a certain angle between the two jets.

Pinnacle is developing a multicylinder 1.2 L gasoline engine for automotive applications using high-performance computing (HPC) and analysis methods. Pinnacle and Oak Ridge National Laboratory executed large-scale multidimensional combustion analyses at the Oak Ridge Leadership Computing Facility to thoroughly explore the design space. These HPC-led investigations show high fuel efficiency (∼46% gross indicated efficiency) may be achieved by operating with extremely high charge dilution levels of exhaust gas recirculation (EGR) at a light load key drive cycle condition (2000 RPM, 3 bar brake mean effective pressure (BMEP)), while simultaneously attaining high levels of fuel conversion efficiency and low NO x emissions. In this extremely dilute environment, the flame propagation event is supported by turbulence and bulk in-cylinder charge motion brought about by modulation of inlet port flow. This arrangement produces a load and speed adjustable amalgamation of swirl and counter-rotating tumble which provides the turbulence required to support stable low-temperature combustion. At higher load conditions, the engine may operate at more traditional combustion modes to generate competitive power. In this paper, the numerical results from these HPC simulations are presented. Further HPC simulations and test validations are underway and will be reported in future publications.

We systematically determine the maximally efficient manner of using water and air in a single-cycle steady-flow combustion gas turbine power plant. In doing so, we identify the upper limit to exergy efficiency for dry and wet gas turbine engines through architectures that employ regenerative work, heat, and matter transfers using imperfect practical devices. For existing device technology, the derived optimal architectures can theoretically achieve exergy efficiency above 65% without employing a bottoming cycle. This surpasses known efficiencies for both wet and combined cycles. We also show that when optimally used, nonreactive matter transfers, like water, provide an alternative, but not superior, thermal regeneration strategy to direct heat regeneration.

In-cylinder air flow structure makes significant impacts on fuel spray dispersion, fuel mixture formation, and flame propagation in spark ignition direct injection (SIDI) engines. While flow vortices can be observed during the early stage of intake stroke, it is very difficult to clearly identify their transient characteristics because these vortices are of multiple length scales with very different swirl motion strength. In this study, a high-speed time-resolved two-dimensional (2D) particle image velocimetry (PIV) is applied to record the flow structure of in-cylinder flow field along a swirl plane at 30 mm below the injector tip. First, a discretized method using flow field velocity vectors is presented to identify the location, strength, and rotating direction of vortices at different crank angles. The transients of vortex formation and dissipation processes are revealed by tracing the location and motion of the vortex center during the intake and compression strokes. In addition, an analysis method known as the wind-rose diagram, which is implemented for meteorological application, has been adopted to show the velocity direction distributions of 100 consecutive cycles. Results show that there exists more than one vortex center during early intake stroke and their fluctuations between each cycle can be clearly visualized. In summary, this approach provides an effective way to identify the vortex structure and to track the motion of vortex center for both large-scale and small-scale vortices.

The design and development of high efficiency spark-ignition engines continues to be limited by the consideration of knock. Although the topic of spark knock has been the subject of comprehensive research since the early 1900s, little has been reported on the coupling of the engine thermodynamics and knock. This work uses an engine cycle simulation together with a submodel for the knock phenomena to explore these connections. First, the autoignition characteristics as represented by a recent (2014) Arrhenius expression for the reaction time of the end gases are examined for a range of temperatures and pressures. In spite of the exponential dependence on temperature, pressure appears to dominate the ignition time for the conditions examined. Higher pressures (and higher temperatures) tend to enhance the potential for knock. Second, knock is determined as function of engine design and operating parameters. The trends are consistent with expectations, and the results provide a systematic presentation of knock occurrence. Engine parameters explored include compression ratio, engine speed, inlet pressure, start of combustion, heat transfer, and exhaust gas recirculation (EGR). Changes of cylinder pressures and temperatures of the unburned zone as engine parameters were varied are shown to be directly responsible for the changes of the knock characteristics.

A partially premixed combustion (PPC) approach was applied in a single cylinder diesel research engine in order to characterize engine power improvements. PPC is an alternative advanced combustion approach that generally results in lower engine-out soot and oxides of nitrogen (NO x ) emission, with a moderate penalty in engine-out unburned hydrocarbon (UHC) and carbon monoxide (CO) emissions. In this study, PPC is accomplished with a minority fraction of jet fuel injected into the intake manifold, while the majority fraction of jet fuel is delivered directly to the combustion chamber near the start of combustion (SOC). Four compression ratios (CR) were studied. Exhaust emissions plus exhaust opacity and particulate measurements were performed during the experiments in addition to fast in-cylinder combustion metrics. It was seen that as CR increased, the soot threshold equivalence ratio decreased for conventional diesel combustion; however, this afforded an increased opportunity for higher levels of port injected fuel leading to power increases from 5% to 23% as CR increased from 14 to 21.5. PPC allowed for these power increases (defined by a threshold opacity level of 3%) due to smaller particles (and lower overall number of particles) in the exhaust that influence measured opacity less significantly than larger and more numerous conventional diesel combustion exhaust particulates. Carbon monoxide levels at the higher PPC-driven power levels were only modestly higher, although NO x was generally lower due to the overall enriched operation.